The flux-dependent rms variability of X-ray binaries in the optical

A linear relation between absolute rms variability and flux in X-ray observations of compact accreting sources has recently been identified. Such a relation suggests that X-ray lightcurves are non-linear and composed of a lognormal distribution of fluxes. Here, a first investigation of the optical rms vs. flux behavior in X-ray binaries is presented. Fast timing data on three binaries in the X-ray low/hard state are examined. These are XTE J1118+480, GX 339-4 and SWIFT J1753.5-0127 -- all show aperiodic (non-reprocessed) optical fluctuation components. Optical rms amplitude is found to increase with flux in all sources. A linear fit results in a positive offset along the flux axis, for most frequency ranges investigated. The X-ray and optical relation slopes track the source fractional variability amplitudes. This is especially clear in the case of GX 339-4, which has the largest optical variance of the three targets. Non-linearity is supported in all cases by the fact that flux distributions of the optical lightcurves are better described with a lognormal function than a simple gaussian. Significant scatter around linearity is found in the relation for the two sources with lower optical variability amplitude, though observational biases may well contribute to this. Implications for accretion models are discussed, and the need for long well-sampled optical lightcurves is emphasized.